There are many hinge designs used in orthopedic splints and braces employed at the knee.
In a first widely used type, there are two hinge arms joined at and flexing about a single pivot. This type is generally referred to by those skilled in the art as the uni-axial, uni-pivotal or monocentric type.
In a second type, perhaps even more widely used, there are two hinge arms each having its own pivot and also each having a set gear teeth about the periphery of the part which extends between the pivots. The arms are so sized and arranged that the gear teeth mesh between the pivot points thereby integrating the arm movements. Thus if one arm moves, the other must move as well. This type is generally referred to by those skilled in the art as the geared bi-axial, geared duocentric or geared polycentric type. The latter term is perhaps the most widely recognized.
Neither of these hinge types is remotely physiological in the way they move and because their mechanical action is so unlike that of the human knee their use in a brace construct may be positively deleterious to an injured, repaired or deformed knee.
The term brace construct is used by those skilled in the art to describe the resultant mechanical arrangement of a brace and the leg to which it is attached. A combination of casting materials (such as Plaster of Paris or resin impregnated bandages) and cast bracing hinges as well as braces secured on the leg by means such as straps are brace constructs once they are in place on the leg.
Mechanically, the knee is a modified, crossed, four bar linkage comprising the rigid elements femur, tibia and the anterior and the posterior cruciate ligaments. Its axis of rotation moves backwards or posteriorly as the knee is flexed from the fully extended position. The locus or track of the axis of knee rotation is called the "Instant Center Pathway" which exactly defines the moving path of the center of knee rotation at any given instant. Uni-axial and geared polycentric hinges do not have this construction; they do not move in this manner and within a brace construct they cannot accommodate or track the complex motion of the knee properly.
Another type of hinge design used in orthopedic splints and braces employed at the knee has two hinge arms each having its own pivot but in this design there are not gear teeth. Thus, in this type, the arm movements are not integrated and each arm can always move independently without affecting the other. This type of hinge is generally referred to by those skilled in the art as the true bi-axial, true bi-pivotal or simply just bi-pivotal type. It continues to grow in popularity with the realization that such a construction is superior to the others in providing the freedom necessary to accommodate the complex and changing locus of the axis of the knee throughout the entire flexion/extension cycle.
Both bi-pivotal hinges and geared polycentric hinges are, in mechanical terms, three bar linkages. However, the geared integration of the hinge arms in the geared polycentric type arms causes the loss of one degree of freedom. When used in most orthopedic braces both types require at least one stop at or near the fully extended position to prevent over-travel of the knee into hyperextension. In bracing and general orthopedic and orthotic applications a three bar linkage hinge mechanism, with all degrees of freedom available, offers the most practical and appropriate mechanical arrangement for accommodating the complex motion of the knee.
The present author has been concerned for many years with research and development of bi-pivotal hinges based upon pivot spacings between 24 mm and 30 mm. When such hinges are used as part of a brace construct, the net rearward travel of the instant center pathway of the knee axis which can be accommodated approximates to that which is likely to be encountered in the great majority of patients. This was confirmed at Sheffield and Brunel Universities in the United Kingdom in 1986 and 1987.
In addition it was confirmed, in 1984, by means of combined video, computer and force-plate gait analysis, that when used at the knee, bi-pivotal hinges with such pivot spacings introduce less disturbance to the normal gait (or walking pattern), than either geared polycentric two-pivot hinges or uni-axial hinges. Furthermore, it has been shown that pistoning and zig-zagging does not occur in such hinges (provided they are fitted properly) when the knee is under load. This work was carried out at Derby Royal Infirmary, Derby, United Kingdom in 1984 and was presented at the 8th World Orthopaedic Congress in Washington, DC, USA, May 4-10, 1987 by Dr. David Pratt. Dr. Pratt's principal co-author was David Rowley, M.D., F.R.C.S., now Professor of Orthopaedics and Trauma Surgery, The University, Dundee, Scotland.
By way of contrast, if geared polycentric hinges are restrained at a point along each hinge arm some distance from each pivot and flexed, they are driven forwards in the opposite direction to natural knee motion. In a brace construct this is exactly the form of restraint applied, usually by sets of securing straps above and below the knee. This movement is easy to demonstrate in a brace not in place on a leg. However, in a brace construct movement is substantially prevented by the securing straps and the net effect is to generate forces which act counter to the flexing forces generated by the flexor muscle groups acting on the knee. The opposite effect is encountered during extension of the knee in such a brace construct and in both cases, the resolution of these extraneous forces is through a compromised knee joint.
It is possible, by altering the architecture of a geared polycentric hinge, to partially alleviate these effects. This can be done, for instance, by introducing a substantial anterior displacement of the femoral hinge arm and a substantial posterior displacement of the tibial hinge arm and arranging the gearing accordingly. Such an arrangement is taught in U.S. Pat. No. 4,697,583 to Mason et al., assigned to Don Joy Orthopedic Inc. of Carlsbad, Calif., USA.
Another concern to those who have to set up and adjust braces is the total number of parts to be handled when a brace needs adjustment. This is a particular problem with braces which have incremental extension and flexion stop. In every commercially available system known to the present author which employs incremental stops, extension stops and flexion stops are secured separately by at least one screw for each stop and there is often a hinge cover which has to be removed as well. This is the case with a 1995 released product from Don Joy Inc. of Carlsbad, Calif., and called "Legend".TM. which is admittedly a geared polycentric device. In this product, two screws on each hinge release a hinge cover, a flexion stop and an extension stop. To make any change, at least two further stops must be selected and handled from a choice of four different extension stops (8 in total) and four different flexion stops (8 in total).
This calls for a minimum of four screws, two hinge covers and four stops (two outgoing and two incoming)--10 parts plus the selection of two of these from a minimum of eight--to be handled at any given stop change. This level of handling is not particularly unusual in prior art braces but it is very time consuming.
However, the present invention is concerned solely with art of bi-pivotal hinges and further references to the prior art will be so restricted. The present author has been the first inventor of a number of adjustment mechanisms for true bi-pivotal hinges providing continuously variable stops. These are described in patents GB 2 182 714 and U.S. and EPO counterparts U.S. Pat. No. 4,915,098 and 327286, respectively, U.S. Pat. No. 5,000,170 and Canadian counterpart 1 299 455; GB 2 208 065 and U.S. counterpart U.S. Pat. No. 4,881,299 and U.S. Pat. No. 5,039,247.
A series of patents to Borig et al. is relevant to continuously variable stops in bi-pivotal hinges and comprises UK 2 207 458 and counterparts U.S. Pat. No. 4,881,532, EPO 301817, AUS 79761 and SA 88/5584.
In addition, the present author is first inventor of U.S. Pat. No. 5,038,765 which discloses selectable "T"-shaped inserts with abutment stops for limiting angular travel of one movement of both arms of a true bi-pivotal hinge. As generally disclosed, this movement is extension, which is undoubtedly the most usual movement which would be required to be controlled. However, claim 1 is not limited to extension and it is clear that flexion, alone, could also be limited as taught. In such a flexion limiting implementation, separate means for at least preventing hyperextension, would necessarily have to be included.
A brace embodying these inserts for limiting extension travel has been sold for several years, by leading companies, including recently by Johnson and Johnson Professional of Raynham, Mass., and Bracknell, Berkshire, United Kingdom in a number of countries, including the United Kingdom and the United States. The product is sold under the commercial name Masterbrace.TM. and is manufactured by Protectair Limited, Abingdon, Oxfordshire, United Kingdom.
The present author is aware of few other commercially available examples of true bi-pivotal hinges and has found few relevant references in the art via patent and commercial literature searches.
In recent years a great deal of attention has been paid by those skilled in the art to the design of functional knee braces which are intended to provide stability for unstable knees. current opinion favors designs in which hinges are disposed in close approximation to the knee, have minimal thickness profiles and generally offer limited control of extension and flexion.
Accordingly, continuously variable stop mechanisms are now mainly found in rehabilitation braces which are used immediately following injury or repair to knee ligaments and are otherwise generally simpler braces. In those patients at a later stage of rehabilitation and returning to active sport, discontinuous or incremental stop mechanisms are acceptable in the functional knee braces typically prescribed at that stage.
A functional knee brace known to have been made and sold by Messrs. Omni Scientific of Martinez, Calif., is believed to be based on Anderson, U.S. Pat. No. 4,249,524. This teaches a true bi-pivotal hinge; however, the pivots are very widely spaced. It seems inevitable that, in such a hinge, shortening would occur during flexion, leading to effective shortening of a cast or brace in which it was used. Such shortening would allow the injured or recently repaired knee joint to piston and to experience undesirable loads.
In the Anderson patent, the hinge center bar apparently fulfills the function of both a mounting for the pivots and for the hinge arms since it extends to a position where members, normally termed headplates or limb bands, would be employed.
For instance, in a functional knee brace intended for use by a person returning to active contact sport following a ligament injury, there is an expectation, not always justified, that the brace will provide physical protection for the knee. If wide pivotal spacing is employed on the lateral hinge, the medial collateral ligament will receive little, if any, protection from the brace if a substantial, medially directed, lateral blow is suffered when the knee is moderately flexed.
Although Anderson briefly mentions stops, no motion control system is disclosed in any detail. In any such system, the inter-relationship between the control of motion and the pivot spacing in true bi-pivotal hinges is of paramount importance.
Directly relevant is U.S. Pat. No. 4,520,802 to Mercer and Aaserude which teaches another bi-pivotal cast bracing hinge featuring wide pivot spacing. These authors disclose a motion control system based preferably upon multiple indexing blocks.
As written, the intention seems to be to provide mainly flexion control for a true bi-pivotal hinge on at least one flex bar (frequently termed a hinge arm by other authors). FIGS. 1 and 5 support this view since it is clear in these drawings that extension is limited only by the fundamental provision of hyperextension stop means represented by element 23. The specification is confusing in that element 5 is described as a support bar or rod but in FIGS. 1 and 4 it appears to be illustrated as a screw or pin structure which might be means of limiting extension but which is not described as such.
Dispositions of the indexing blocks are disclosed both beyond and between the pivots although claim 1 is limited to the latter position. It is certainly not possible to envisage any method or means within the scope of this disclosure by which more than one flex bar could be controlled by one index block if it were disposed beyond one pivot. It is also very difficult to envisage how closely spaced pivots could be employed if two index blocks were disposed between the pivots.
The detailed description states, in column 6 at line 27, a preference for a pair of index blocks, thereby contemplating the use of a single structure. No means is actually taught for accomplishing this and it is neither clear nor obvious from any part of the disclosure how this might be achieved. In claim 1, the authors refer to a structure involving a pair of index blocks and it is clear that these are intended to be used together and disposed between the pivots.
For instance, in FIG. 1, there is shown an arrangement of two indexing blocks disposed between the pivots, necessitating a minimum distance between the pivots of two indexing block widths, plus an allowance for the ends of the flex bars sufficient to create a flexion stop on each and leaving sufficient metal about the pivots to ensure structural integrity and durability. In such a case, the argument applied to wide spacing in relation to the Anderson patent op cit applies. In FIG. 5, where two indexing blocks are disposed outside the pivots, there is no need for the wide spacing shown but one block could not control both pivots.
In addition, it seems clear from drawings 1 and 5b and the detailed description, that the authors did not contemplate the provision of both flexion stops and extension stops on one indexing block. It also seems at least likely that the means for providing extension was intended to be conveyed by a description of element 5, the description of which as a "support bar or rod" duplicates the description of element 1.
In the structure as disclosed, it would be impossible to fit a single indexing block, even with extension stops, since the broadest dimension of such a block would have to be introduced through a narrower dimension between those portions of the flex arms extending between the pivots, in order to reach and act upon on the extension surfaces of the flex bars which come to rest against the back of center bar element 23.
With two indexing blocks, intended to offer extension stops, the pivot spacing would have to be widened still further to allow introduction of both blocks and this would, in any case, be physically difficult. Once again the general argument applied to wide spacing would detract from the usefulness of such a hinge.
FIG. 3, in conjunction with the paragraph commencing at line 36 of column 6 seems to indicate clearly the authors' intentions, namely that the second class of face designated 14, abuts the back of center bar 23 (FIG. 1) locking index block 3 in position whichever flexion stop face is selected. If this condition is fulfilled, it becomes geometrically impossible to incorporate flexion stop faces and extension stop faces together within a single index block 3. This applies regardless of whether one or two are used, in any rational orthopedic or orthotic hinge device. The provision of structural element 23 appears crucial to the implementation of index blocks as taught by Mercer and Aaserude.